Attenuation of engine harshness during lean-to rich transitions

ABSTRACT

Engine systems and methods for accomplishing regeneration of a NOx adsorber ( 28 ) using in-cylinder post-injection in a way that creates a lean-rich transition (FIG.  3 ) for regenerating the NOx adsorber while attenuating engine torque output fluctuations during the transition without the necessity of using torque sensing to attenuate the fluctuations.

FIELD OF THE INVENTION

This invention relates to motor vehicles that are powered by internalcombustion engines whose operation may for any of various reasonstemporarily transition from running lean to running rich, one reasonbeing to purge a NOx adsorber in the engine exhaust system of adsorbedNOx so that it can continue to be effective as the engine continuesrunning. More particularly, the invention relates to systems and methodsfor attenuating fluctuations in engine output torque that contribute toharness in the operation of such vehicles during lean-rich transitionssuch as those for regenerating a NOx adsorber.

BACKGROUND OF THE INVENTION

An exhaust system of a diesel engine that comprises a NOx adsorber iscapable of adsorbing significant amounts of NOx in exhaust gas passingthrough the exhaust system from the engine, thereby reducing the amountof NOx that otherwise would enter the atmosphere. From time to time,such a device must be regenerated in order to purge it of adsorbed NOxso that it can continue to be effective in adsorbing NOx as the enginecontinues to run. A known technique for regenerating a NOx adsorbercomprises creating an excess of CO for reaction with adsorbed NOx toreduce the NOx to molecular nitrogen (N₂) while the CO oxidizes to CO₂during the process.

One known method for creating excess CO comprises injecting fuel inproper amount into the exhaust leaving engine combustion chambers.Because that fuel does not contribute to the thermal energy ofcombustion that is converted by thermodynamic processes in thecombustion chambers acting on the engine's kinematic mechanism to createengine torque, it has essentially no influence on engine torque.

For one or more reasons, post-injection of fuel that does contribute tothe thermal energy of combustion that produces engine torque may beconsidered a more desirable alternative, although both methods requirethe injection of extra fuel to purge the NOx adsorber. However, thepost-injection alternative has consequences on engine torque output thatcan lead to undesirable torque fluctuations that contribute to engineand vehicle harshness as the engine continues to run during NOx adsorberregeneration.

A known electronic engine control system comprises a processor-basedengine controller that processes data from various sources to developcontrol data for controlling certain functions of the engine. The amountand the timing of engine fueling are two functions that are controlledby an engine control system. A typical diesel engine that comprises fuelinjectors for injecting fuel into the engine cylinders under control ofan engine control system controls both the duration and the timing ofeach fuel injection to set both the amount and the timing of enginefueling. During an engine cycle, it is also capable of pre-injection offuel (pilot-injection) in advance of a main injection and post-injectionafter the main injection, although the use of either typically dependson how the engine is being operated.

SUMMARY OF THE INVENTION

The present invention relates to engine systems and methods foraccomplishing regeneration of a NOx adsorber using in-cylinderpost-injection in a way that creates a lean-rich transition forregenerating the NOx adsorber while attenuating engine torque outputfluctuations during the transition without the necessity of using torquesensing to attenuate the fluctuations.

Accordingly, one generic aspect of the present invention relates to amethod for control of output torque developed by an internal combustionengine during lean-rich modulation of engine operation. With the enginerunning lean, data values of certain parameters are processed to developa data value for desired engine fueling for causing the engine todevelop a corresponding desired output torque at a given engine speed.

As the engine operation changes from running lean to running rich,engine output torque is maintained substantially at the correspondingdesired output torque at the given engine speed by processing i) thedata value for desired engine fueling resulting from the processing ofcertain parameters, ii) a data value for engine speed, and iii) a datavalue for actual air-fuel ratio at which the engine is operating, tothereby develop a data value for desired engine fueling for causing theengine to run rich while striving to maintain engine output torque atthe corresponding desired output torque at the given engine speed whenthe engine was running lean.

Another generic aspect relates to an engine incorporating a controlstrategy for implementing the foregoing generic method.

Still another generic aspect relates to a method for regenerating a NOxadsorber in an exhaust system of an internal combustion engine that isfueled in accordance with a data value for desired engine fueling. Themethod comprises processing data values of certain parameters to developa data value for desired engine fueling for causing the engine todevelop a corresponding desired output torque without conditioningengine exhaust passing into the exhaust system for NOx adsorberregeneration.

Data values of various parameters indicative of conditions relevant toinitiation of NOx adsorber regeneration are processed and after thatprocessing has disclosed that NOx adsorber regeneration can beinitiated, NOx adsorber regeneration is initiated by changing enginefueling so as to condition engine exhaust passing into the exhaustsystem for regenerating the NOx adsorber.

The method develops a data value for desired engine fueling that iseffective to condition the exhaust gas for NOx adsorber regeneration ata given engine speed while striving to maintain the output torque at thecorresponding output torque that desired engine fueling would develop atthe given engine speed without conditioning engine exhaust passing intothe exhaust system for NOx adsorber regeneration. This is accomplishedby processing data values for i) the desired engine fueling that woulddevelop that corresponding output torque without conditioning engineexhaust passing into the exhaust system for NOx adsorber regeneration,ii) engine speed, and iii) actual air-fuel ratio at which the engine isoperating.

The foregoing, along with further features and advantages of theinvention, will be seen in the following disclosure of a presentlypreferred embodiment of the invention depicting the best modecontemplated at this time for carrying out the invention. Thisspecification includes drawings, now briefly described as follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general schematic diagram of portions of a diesel enginerelevant to the present invention.

FIG. 2 is a schematic diagram of a portion of control strategy for theengine.

FIG. 3 is a graph plot showing time traces of several parametersrelevant to engine operation.

FIG. 4 is a first graph plot useful in understanding principles of theinvention.

FIG. 5 is a second graph plot useful in understanding principles of theinvention.

FIG. 6 is a schematic diagram of another portion of the engine controlstrategy.

FIG. 7 is a schematic diagram of another portion of the engine controlstrategy.

FIG. 8 is a third graph plot useful in understanding principles of theinvention.

FIG. 9 is a fourth graph plot useful in understanding principles of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a schematic diagram of an exemplary diesel engine 20 forpowering a motor vehicle. Engine 20 has a processor-based engine controlsystem 22 that processes data from various sources to develop variouscontrol data for controlling various aspects of engine operation. Thedata processed by control system 22 may originate at external sources,such as sensors, and/or be generated internally.

Control system 22 controls the operation of electric-actuated fuelinjectors that inject fuel into engine combustion chambers. A processorof control system 22 can process data sufficiently fast to calculate, inreal time, the timing and duration of injector actuation to set both thetiming and the amount of fueling. The injection process comprises a maininjection, and under certain conditions, a pilot injection and/or apost-injection. Control system 22 calculates a data value VF_des_m thatrepresents the amount of fuel that is to be injected into a combustionchamber during an engine cycle.

Engine 20 further comprises an intake system 24 through which charge airenters the combustion chambers, and an exhaust system 26 through whichexhaust gases resulting from combustion leave the engine. Exhaust system26 includes a NOx adsorber 28 that adsorbs significant amounts of NOx inexhaust gas passing from engine 26, thereby reducing the amount of NOxthat otherwise would enter the atmosphere.

From time to time, NOx adsorber 28 must be regenerated in order to purgeit of adsorbed NOx so that it can remain effective as the enginecontinues to run. A known technique for regenerating a NOx adsorbercomprises creating an excess of CO for reaction with adsorbed NOx toreduce the NOx to molecular nitrogen (N₂) while the CO oxidizes to CO₂during the process.

FIG. 2 discloses a strategy 30 that is executed by control system 22 todetermine when regeneration can be performed. The strategy is premisedon the general factors: exhaust gas temperature; elapse of time sincethe previous regeneration; and driveability of the vehicle.

Temperature of exhaust gas proximate the inlet of NOx adsorber 28,obtained by either estimation or measurement, is represented by the datavalue for a parameter ADS_INLET_T. Temperature of exhaust gas proximatethe outlet of NOx adsorber 28, obtained by either estimation ormeasurement, is represented by the data value for a parameterADS_OUTLET_T. The data value for ADS_INLET_T is compared by a comparisonfunction 32 with a data value INLET_T_LL representing a lowertemperature limit at or above which it would be appropriate toregenerate NOx adsorber 28. The data value for ADS_OUTLET_T is comparedby a comparison function 34 with a data value OUTLET_T_LL representing alower temperature limit at or above which it would be appropriate toregenerate NOx adsorber 28.

If either comparison function 32, 34 is satisfied by the correspondingactual temperature being equal to or greater than the respective lowerlimit, then a logical OR function 36 enables regeneration to proceed.Additional conditions must also be satisfied however before regenerationactually proceeds.

Time elapsed since the last regeneration is measured by a timer function38. The data value for elapsed time is compared by a comparison function40 with the data value for a minimum interval between regenerations.Once elapsed time equals or exceeds the minimum, regeneration isenabled. An AND logic function 42 assures that both a temperatureminimum and a time minimum have been satisfied before regeneration isenabled.

Once strategy 30 has been enabled, actual regeneration becomes afunction of driveability. Driveability refers to acceptable vehiclevibration and harshness during lean/rich transition. A torque lowerlimit and upper limit have been set to minimize vibration and harshnessduring the transition. The lower limit requires the engine to be runningwith some load, while the upper limit keeps the engine away from severeacceleration conditions.

The data value for a parameter TORQUE represents the torque which engine20 is producing. TORQUE is the estimated torque based on fueling andengine speed, in other words TORQUE=f(VFDES, N). The data value forTORQUE is compared by a comparison function 44 with a data valueTORQUE_UL representing an upper torque limit above which regenerationwould be inappropriate. The data value for TORQUE is compared by acomparison function 46 with a data value TORQUE_LL representing a lowertorque limit below which regeneration would be inappropriate. An ANDlogic function 48 processes the results of both comparisons to assurethat torque is within the allowable range for NOx adsorber regeneration.

A further AND function 50 processes outputs from both AND functions 42,48 to allow regeneration when the three general factors of exhaust gastemperature, elapse of time since the previous regeneration, anddriveability of the vehicle are satisfied.

In general, a diesel engine runs cooler, slower, and leaner than aspark-ignition engine. During lean running, engine 20 generates NOx thatis adsorbed by NOx adsorber 28. When the adsorber is to be regenerated,engine operation transitions from running lean to running rich in orderto condition the exhaust for purging NOx adsorber 28 by generating theneeded excess CO. A trace 60 in FIG. 3 represents air-fuel ratio. Beforetime to engine 20 is running lean, the NOx loading of NOx adsorber 28,represented by a trace 62, is increasing, and CO concentration,represented by a trace 64, is relatively low.

At time t₁ post injection and air management decrease the air-fuelratio, creating a surge in CO concentration in the process. At time t₂lean running resumes. Trace 62 shows that the surge is effective topurge NOx adsorber 28 of a significant amount of its NOx load.

The conditions portrayed by FIG. 3 assume that certain inputs to controlsystem 22, namely engine speed and accelerator pedal position arecommanding engine 20 to develop a substantially constant torque. Becausethe regeneration process alters engine fueling from that which isotherwise being called for by engine speed and accelerator pedalposition, engine torque may fluctuate during NOx adsorber regeneration,as represented by a perturbation 70 in a trace of engine torque 72. Asignificant perturbation can contribute to harshness in engine operationthat is consequently introduced into the vehicle drivetrain. It istoward attenuating such harshness that the present invention isdirected.

FIG. 4 shows a trace 80 of engine torque versus air-fuel ratio where,for a given engine speed, the torque remains substantially constant. Atrace 82 shows the corresponding fueling that engine 20 needs in orderto develop the torque represented by trace 80.

Principles of the invention resulted from the recognition that dataclosely approximating trace 82 can be developed by suitable dataprogramming of, and data processing by, control system 22, and theresulting data processed with other data to create desired enginefueling data that, for a given engine speed and desired engine outputtorque, can fuel the engine during a lean-to-rich transition that causesthe engine to run rich while striving to maintain engine output torqueat the corresponding desired output torque at the given engine speedwhen the engine was running lean, thereby attenuating undesiredfluctuations in engine torque that would be experienced in the absenceof the invention.

FIG. 5 shows a piecewise linear approximation 90 of trace 82 to comprisea first linear segment 92 extending between data points marked AFR^(—)rand AFR_C and a second linear segment 94 extending between data pointsmarked AFR_c and AFR_1. AFR symbolizes air-fuel ratio.

Segment 92 can be defined by the functionVF _(—) des _(—) m=αVF _(—) des _(—) m _(—) c+(1−α)VF _(—) des _(—) m_(—) r

-   -   and    -   segment 94 by the function        VF _(—) des _(—) m=βVF _(—) des _(—) m_1+(1−β)VF _(—) des _(—) m        _(—) c        where        α=(AFR−AFR _(—) r)/(AFR _(—) c−AFR _(—) r)        and        β=(AFR−AFR _(—) c)/(AFR_1−AFR _(—) c).

These functional relationships define a control algorithm for desiredengine fueling over a range of air-fuel ratios that will cause engine 20to develop substantially constant torque, although it is to beappreciated that the engine may not necessarily operate all such ratios.In order to generate the excess CO needed for NOx adsorber regeneration,engine 20 needs to run at an AFR below stoichiometric (an AFR ofapproximately 13). Hence for a given torque, a fueling transition fromlean to rich that strives to maintain that torque will take place alongsegment 92. As can be appreciated, the specific parameters for atransition will be governed by a specific regeneration strategy for aparticular engine.

Implementation of the control algorithm in control system 22 isaccomplished by entering data values for AFR_r, AFR_c, and AFR_1 foreach pair of data values for engine torque and engine speed. Asufficient number of pairs of such torque and speed data values areprogrammed into control system 22 to adequately cover the range ofengine operation with sufficient resolution within the range.

For given data values of torque and speed representing current enginetorque and current engine speed, control system 22 operates to selectfrom the closest pair of torque and speed data values that have beenprogrammed into it, the corresponding data values for AFR_r, AFR_c, andAFR_1 for use in calculating a data value for VF_des_m. A data value forthe variable AFR is obtained in any suitably appropriate way andprocessed according to the algorithm to develop the data value forVF_des_m. The processing occurs sufficiently fast in real time to allowvariables like AFR to be updated fast enough to follow changing engineoperation.

The algorithm develops desired fueling data values in the mannerrepresented by FIG. 7. For the selected pair of torque and speed datavalues, control system 22 determines whether actual AFR is above orbelow the AFR represented by the corresponding break point AFR_c (step100 in FIG. 7).

If AFR is greater than AFR_c, then desired fueling is controlled byVF _(—) des _(—) m=βVF _(—) des _(—) m_1+(1−β)VF _(—) des _(—) m _(—) c

-   -   corresponding to step 102 in FIG. 7.

If AFR is equal to or less than AFR_c, then desired fueling iscontrolled byVF _(—) des _(—) m=αVF _(—) des _(—) m _(—) c+(1−α)VF _(—) des _(—) m_(—) r

-   -   corresponding to step 104 in FIG. 7.

When the actual AFR is other than AFR_r, AFR_C, and AFR_1, the algorithmcalculates the data value for desired engine fueling by what amounts tointerpolation, as graphically portrayed by the function 110 in FIG. 8for α, and the function 120 in FIG. 9 for β. The implementation incontrol system 22 is represented by FIG. 6.

Data values for engine speed (parameter N) and accelerator pedalposition (parameter APS) determine, via a map or look-up table 130, adata value for engine output torque (parameter Torque_des). Controlsystem 22 processes Torque_des according to an operating strategy forcausing engine 20 to develop that torque at that speed. When NOxadsorber 28 is not being regenerated, it is Torque_des that controlsdesired engine fueling by a different portion of the strategy that isnot shown here. When regeneration is occurring, Torque_des is still afactor in controlling desired engine fueling, but not the sole factorbecause the control algorithm that is used during regeneration takes AFRinto account.

Another look-up table 132 contains data values for VF_des_c correlatedwith the pair of the data value for engine speed (N) and the data valuefor Torque_des that would be essentially exclusively controlling desiredengine fueling if regeneration were not occurring. Still another look-uptable 134 contains data values for VF_des_r correlated with the pair ofthe data value for engine speed (N) and the data value for Torque_desthat would be essentially exclusively controlling desired engine fuelingif regeneration were not occurring.

Data values for N and Torque_des determine, via maps 132 and 134, datavalues for VF_des_m_c (representing desired fueling when AFR=AFR_c) andVF_des_m_r (representing desired fueling when AFR=AFR_r). The controlalgorithm then utilizes those data values for its calculation. Becausedata values for N and Torque_des can change during regeneration, theexecution rate of the control algorithm is sufficiently fast to followthose changes so that data values for VF_des_m_c and VF_des_m_r can behanged accordingly as called for by the maps.

While a presently preferred embodiment of the invention has beenillustrated and described, it should be appreciated that principles ofthe invention apply to all embodiments falling within the scope of thefollowing claims.

1. A method for control of output torque developed by an internalcombustion engine during lean-rich modulation of engine operation, themethod comprising: a) with the engine running lean, processing datavalues of certain parameters to develop a data value for desired enginefueling for causing the engine to develop a corresponding desired outputtorque at a given engine speed; b) causing the engine to transition fromrunning lean to running rich while striving to maintain engine outputtorque at the corresponding desired output torque at the given enginespeed by processing i) the data value for desired engine fuelingresulting from the processing of step a), ii) a data value for enginespeed, and iii) a data value for actual air-fuel ratio at which theengine is operating, to thereby develop a data value for desired enginefueling that causes the engine to run rich while striving to maintainengine output torque at the corresponding desired output torque at thegiven engine speed when the engine was running lean.
 2. A method as setforth in claim 1 in which step b) comprises: with the engine runningrich, processing a data value for actual air-fuel ratio through afunction that correlates data values of air-fuel ratio to data values ofengine fueling for causing the engine to run rich while striving tomaintain engine output torque at the corresponding desired output torqueat the given engine speed when the engine was running lean, and causingthe data value of desired engine fueling to be the data value of enginefueling correlated by the function to the data value of actual air-fuelratio.
 3. A method as set forth in claim 1 wherein step a) comprises:with the engine running lean, processing data values of engine speed andaccelerator pedal position to develop a data value for desired enginefueling for causing the engine to develop a corresponding desired outputtorque at a given engine speed.
 4. An internal combustion enginecomprising: a fueling system for fueling the engine in accordance with adata value for desired engine fueling; and a control system forprocessing various data to develop data for control of various enginefunctions including data values for desired engine fueling, wherein thecontrol system comprises a control strategy a) for causing the engine torun lean, and with the engine running lean, for processing data valuesof certain parameters to develop a data value for desired engine fuelingfor causing the engine to develop a corresponding desired output torqueat a given engine speed; and b) for causing the engine to transitionfrom running lean to running rich while striving to maintain engineoutput torque at the corresponding desired output torque at the givenengine speed by processing i) the data value for desired engine fuelingresulting from the processing of a), ii) a data value for engine speed,and iii) a data value for actual air-fuel ratio at which the engine isoperating, to thereby develop a data value for desired engine fuelingthat causes the engine to run rich while striving to maintain engineoutput torque at the corresponding desired output torque at the givenengine speed when the engine was running lean.
 5. An engine as set forthin claim 4 in which the portion of the control strategy for causing theengine to transition from running lean to running rich while striving tomaintain engine output torque at the corresponding desired output torqueat the given engine speed by processing i) the data value for desiredengine fueling resulting from the processing of a), ii) a data value forengine speed, and iii) a data value for actual air-fuel ratio at whichthe engine is operating, to thereby develop a data value for desiredengine fueling that causes the engine to run rich while striving tomaintain engine output torque at the corresponding desired output torqueat the given engine speed when the engine was running lean, comprises:with the engine running rich, processing a data value for actualair-fuel ratio through a function that correlates data values ofair-fuel ratio to data values of engine fueling for causing the engineto run rich while striving to maintain engine output torque at thecorresponding desired output torque at the given engine speed when theengine was running lean, and causing the data value of desired enginefueling to be the data value of engine fueling correlated by thefunction to the data value of actual air-fuel ratio.
 6. An engine as setforth in claim 4 in which the portion of the control strategy forcausing the engine to run lean, and with the engine running lean, forprocessing data values of certain parameters to develop a data value fordesired engine fueling for causing the engine to develop a correspondingdesired output torque at a given engine speed, comprises: with theengine running lean, processing data values of engine speed andaccelerator pedal position to develop a data value for desired enginefueling for causing the engine to develop a corresponding desired outputtorque at a given engine speed.
 7. A method for regenerating a NOxadsorber in an exhaust system of an internal combustion engine that isfueled in accordance with a data value for desired engine fueling, themethod comprising: a) processing data values of certain parameters todevelop a data value for desired engine fueling for causing the engineto develop a corresponding desired output torque without conditioningengine exhaust passing into the exhaust system for NOx adsorberregeneration; b) processing data values of various parameters indicativeof conditions relevant to initiation of NOx adsorber regeneration; andc) after the processing of step b) has disclosed that NOx adsorberregeneration can be initiated, initiating NOx adsorber regeneration bychanging engine fueling so as to condition engine exhaust passing intothe exhaust system for regenerating the NOx adsorber, includingdeveloping a data value for desired engine fueling that is effective tocondition the exhaust gas for NOx adsorber regeneration at a givenengine speed while striving to maintain the output torque at thecorresponding output torque that desired engine fueling would develop atthe given engine speed without conditioning engine exhaust passing intothe exhaust system for NOx adsorber regeneration, by processing datavalues for i) the desired engine fueling that would develop thatcorresponding output torque without conditioning engine exhaust passinginto the exhaust system for NOx adsorber regeneration, ii) engine speed,and iii) actual air-fuel ratio at which the engine is operating.
 8. Amethod as set forth in claim 7 in which the step of developing a datavalue for desired engine fueling that is effective to condition theexhaust gas for NOx adsorber regeneration at the given engine speedwhile striving to maintain the output torque at the corresponding outputtorque that desired engine fueling would develop at the given enginespeed without conditioning engine exhaust passing into the exhaustsystem for NOx adsorber regeneration, comprises: for each of multipledata values of desired engine fueling that do not condition engineexhaust passing into the exhaust system for NOx adsorber regeneration atthe given engine speed, developing data values defining a range ofrelatively smaller air-fuel ratios below relatively larger air-fuelratios, and within that range, a functional relationship between datavalues of air-fuel ratio and data values of desired engine fuelingeffective to condition the exhaust gas for NOx adsorber regenerationwhile striving to maintain the output torque at the corresponding outputtorque that desired engine fueling would develop at the given speedwithout conditioning engine exhaust passing into the exhaust system forNOx adsorber regeneration.
 9. A method as set forth in claim 8 in whichthe step of developing data values defining a range of relativelysmaller air-fuel ratios below relatively larger air-fuel ratios, andwithin the range of relatively smaller air-fuel ratios, a functionalrelationship between data values of the air-fuel ratio and data valuesof desired engine fueling effective to condition the exhaust gas for NOxadsorber regeneration comprises: i) developing a boundary data value ofair-fuel ratio that defines an upper limit of the relatively smallerrange, and for that boundary data value of air-fuel ratio, acorresponding data value of desired engine fueling, and ii) within therange of relatively smaller air-fuel ratios, developing a data value ofair-fuel ratio less than the boundary data value of air-fuel ratio, andfor that data value of air-fuel ratio less than the boundary data valueof air-fuel ratio, a corresponding data value of desired engine fueling.10. A method as set forth in claim 8 in which the step of developing adata value for desired engine fueling that is effective to condition theexhaust gas for NOx adsorber regeneration while striving to maintain theoutput torque at the corresponding output torque that desired enginefueling would develop at the given speed without conditioning engineexhaust passing into the exhaust system for NOx adsorber regeneration byprocessing data values for i) the desired engine fueling that does notcondition engine exhaust passing into the exhaust system for NOxadsorber regeneration, ii) engine speed, and iii) actual air-fuel ratioat which the engine is operating comprises: processing the data valuefor desired engine fueling resulting from step a) to select thecorresponding functional relationship between data values of theair-fuel ratio and data values of desired engine fueling effective tocondition the exhaust gas for NOx adsorber regeneration, and processingthe data value for actual air-fuel ratio at which the engine isoperating according to the selected corresponding functionalrelationship between data values of the air-fuel ratio and data valuesof desired engine fueling effective to condition the exhaust gas for NOxadsorber regeneration, to thereby provide a data value for desiredengine fueling.
 11. A method as set forth in claim 7 wherein step a)comprises: processing data values of engine speed and accelerator pedalposition to develop the data value for desired engine fueling forcausing the engine to develop a corresponding desired output torquewithout conditioning engine exhaust passing into the exhaust system forNOx adsorber regeneration.
 12. A method as set forth in claim 7 whereinstep b) further comprises: conditioning initiation and continuation ofNOx adsorber regeneration on an estimate of engine torque being greaterthan a lower torque limit and less than an upper torque limit.
 13. Aninternal combustion engine comprising: an exhaust system comprising aNOx adsorber for adsorbing NOx from exhaust resulting from combustion offuel in the engine; a fueling system for fueling the engine inaccordance with a data value for desired engine fueling; and a controlsystem for processing various data to develop data for control ofvarious engine functions including data values for desired enginefueling, wherein the control system comprises a control strategy a) forprocessing data values of certain parameters to develop a data value fordesired engine fueling for causing the engine to develop a correspondingdesired output torque without conditioning engine exhaust passing intothe exhaust system for NOx adsorber regeneration, b) for processing datavalues of various parameters indicative of conditions relevant toinitiation of NOx adsorber regeneration; and c) after the processing ofb) has disclosed that NOx adsorber regeneration can be initiated, forinitiating NOx adsorber regeneration by changing engine fueling so as tocondition engine exhaust passing into the exhaust system forregenerating the NOx adsorber, including developing a data value fordesired engine fueling that is effective to condition the exhaust gasfor NOx adsorber regeneration at a given engine speed while striving tomaintain the output torque at the corresponding output torque thatdesired engine fueling would develop at the given engine speed withoutconditioning engine exhaust passing into the exhaust system for NOxadsorber regeneration, by processing data values for i) the desiredengine fueling that would develop that corresponding output torquewithout conditioning engine exhaust passing into the exhaust system forNOx adsorber regeneration, ii) engine speed, and iii) actual air-fuelratio at which the engine is operating.
 14. An engine as set forth inclaim 13 in which the portion of the control strategy for developing adata value for desired engine fueling that is effective to condition theexhaust gas for NOx adsorber regeneration at the given engine speedwhile striving to maintain the output torque at the corresponding outputtorque that desired engine fueling would develop at the given enginespeed without conditioning engine exhaust passing into the exhaustsystem for NOx adsorber regeneration, comprises strategy that: for eachof multiple data values of desired engine fueling that do not conditionengine exhaust passing into the exhaust system for NOx adsorberregeneration at the given engine speed, develops data values defining arange of relatively smaller air-fuel ratios below relatively largerair-fuel ratios, and within that range, a functional relationshipbetween data values of air-fuel ratio and data values of desired enginefueling effective to condition the exhaust gas for NOx adsorberregeneration while striving to maintain the output torque at thecorresponding output torque that desired engine fueling would develop atthe given speed without conditioning engine exhaust passing into theexhaust system for NOx adsorber regeneration.
 15. An engine as set forthin claim 14 in which the portion of the control strategy for developingdata values defining a range of relatively smaller air-fuel ratios belowrelatively larger air-fuel ratios, and within the range of relativelysmaller air-fuel ratios, a functional relationship between data valuesof the air-fuel ratio and data values of desired engine fuelingeffective to condition the exhaust gas for NOx adsorber regenerationcomprises strategy for: i) developing a boundary data value of air-fuelratio that defines an upper limit of the relatively smaller range, andfor that boundary data value of air-fuel ratio, a corresponding datavalue of desired engine fueling, and ii) within the range of relativelysmaller air-fuel ratios, developing a data value of air-fuel ratio lessthan the boundary data value of air-fuel ratio, and for that data valueof air-fuel ratio less than the boundary data value of air-fuel ratio, acorresponding data value of desired engine fueling.
 16. An engine as setforth in claim 14 in which the portion of the control strategy fordeveloping a data value for desired engine fueling that is effective tocondition the exhaust gas for NOx adsorber regeneration at the givenengine speed while striving to maintain the output torque at thecorresponding output torque that desired engine fueling would develop atthe given engine speed without conditioning engine exhaust passing intothe exhaust system for NOx adsorber regeneration by processing datavalues for i) the desired engine fueling that does not condition engineexhaust passing into the exhaust system for NOx adsorber regeneration,ii) engine speed, and iii) actual air-fuel ratio at which the engine isoperating comprises strategy for: processing the data value for desiredengine fueling resulting from a) to select the corresponding functionalrelationship between data values of the air-fuel ratio and data valuesof desired engine fueling effective to condition the exhaust gas for NOxadsorber regeneration, and processing the data value for actual air-fuelratio at which the engine is operating according to the selectedcorresponding functional relationship between data values of theair-fuel ratio and data values of desired engine fueling effective tocondition the exhaust gas for NOx adsorber regeneration, to therebyprovide a data value for desired engine fueling.
 17. An engine as setforth in claim 13 in which the portion of the control strategy forprocessing data values of certain parameters to develop a data value fordesired engine fueling for causing the engine to develop a correspondingdesired output torque without conditioning engine exhaust passing intothe exhaust system for NOx adsorber regeneration comprises strategy for:processing data values of engine speed and accelerator pedal position todevelop the data value for desired engine fueling for causing the engineto develop a corresponding desired output torque without conditioningengine exhaust passing into the exhaust system for NOx adsorberregeneration.
 18. An engine as set forth in claim 13 in which theportion of the control strategy for processing data values of variousparameters indicative of conditions relevant to initiation of NOxadsorber regeneration comprises strategy for conditioning initiation andcontinuation of NOx adsorber regeneration on an estimate of enginetorque being greater than a lower torque limit and less than an uppertorque limit.